问题
I am inserting a key key and a value val into the tree-map t returning a new tree with a node containing the key and the value in the appropriate location.
I have this defined:
(define (tree-node key value left right)(list key value left right))
(define (get-key tn) (node_key tn))
(define (get-val tn) (node_val tn))
(define (get-left tn) (node_left tn))
(define (get-right tn) (node_right tn))
(define (node_key tn) (list-ref tn 0))
(define (node_val tn) (list-ref tn 1))
(define (node_left tn) (list-ref tn 2))
(define (node_right tn) (list-ref tn 3))
Im trying this insert method
(define (insert t key val)
(cond (empty? t))
(tree-node (key val '() '()))
((equal key (get-key t))
(tree-node (key val (get-left t) (get-right t)))
((<key (get-key t))
(tree-node ((get-key t) (get-val t) (insert (get-left t key val)) (get-right t)))
(tree-node ((get-key t) (get-val t) (get-left t) (insert (get-right t key val)))))))
I'm using this to define
(define right (insert (insert empty 3 10) 5 20))
and I'm getting this error in DrRacket
application: not a procedure;
expected a procedure that can be applied to arguments
given: (list 0 1 2)
arguments...: [none]
回答1:
What is happening is that a tree-node (which is just a list) is being returned in a position where a function is expected. Scheme cannot execute the list - it's not a function, so it's complaining.
I suspect this is because you have messed up your parentheses.
(cond (empty? t))
Is a self-contained compound expression. It is evaluated, returning the value of t. The expressions after that are not evaluated as part of the cond.
来源：https://stackoverflow.com/questions/12910579/insert-node-key-and-value-in-binary-search-tree-using-scheme